1. Field of the Invention
Embodiments of the present invention relate to detection of electromagnetic signals from hidden conductors and, in particular, to the digital detection of electromagnetic signal strength and signal direction in buried or hidden metallic pipes and cables.
2. Discussion of Related Art
Underground pipe and cable locators (sometimes termed line locators) have existed for many years and are described in many issued patents and other publications. Line locator systems typically include a mobile receiver and a transmitter. The transmitter is coupled to a target conductor, either by direct electrical connection or through induction, to provide a signal on the target conductor. The receiver detects and processes a signal, which can be a continuous wave sinusoidal signal, transmitted from the target conductor as a result of the signal provided to the target conductor by the transmitter.
The transmitter is physically separate from the receiver, often with a separation distance of several meters to several kilometers. The transmitter couples the signal, which can be user-chosen from a selectable set of frequencies, to the target conductor. The frequency of the signal applied to the target conductor can be referred to as the active locate frequency. The target conductor then generates an electromagnetic field in response to the signal.
Different location methodologies and underground environments call for different active frequencies. The typical range of active locate frequencies can be from several Hertz (for location of the underground conductor over separation distances between the transmitter and receiver of many kilometers of cable) to 100 kHz or more. Significant radio frequency interference on the signal received by the receiver can be present in the environment over this range. Therefore, the receivers of line location systems have generally included a set of highly tuned analog filters to preclude interference from outside sources from affecting the measurement of signals at the desired active locate frequency from the target conductor. These filters are tuned to receive signals at each of the selectable active locate frequencies.
Some existing systems include a microprocessor or digital signal processor (DSP) to determine the amplitude of the signal from the target conductor detected at the receiver. For detection of signals generated by the target conductor that are at higher frequency, analog heterodyne methods have been employed in receivers to down-convert segments of the RF spectrum to a lower bandwidth, allowing the digital signal processing to run at lower sample rates while detecting the key physical parameters of the signal used for line location.
Existing receivers for line location systems include analog front-end designs that are tuned to detect signals at the active locate frequency (or frequencies). When more than one active frequency is available in the line locator system, additional analog signal processing elements are often present in the receiver to process signals of each of the available active frequencies. Precise internal calibrations, which are sensitive to drift and other performance degradations over time, are required to process signals at each of the available active frequencies. The calibration process itself is often problematic due to interference and noise sources, including those present in the environment and the receiver itself.
In line location systems, the signal strength parameter (related to the amplitude of the received signal) is the basis for derived quantities of line current, position relative to the center of the conductor, depth of the conductor, and is used as the input to a peak or null indicator (depending on the orientation of the coil antenna). All line location systems measure signal strength on one or more measurement channels. Examples of line locators are included in the following U.S. patents: U.S. Pat. No. 6,130,539, “Automatic Gain Control for a Line Locator,” Stevan Polak, assigned to the same assignee as is the present invention, herein incorporated by reference in its entirety; and U.S. Pat. No. 6,407,550, “Line Locator with Accurate Horizontal Displacement Detection,” Gopalakrishnan Parakulum and Stevan Polak, assigned to the same assignee as is the present invention, herein incorporated by reference in its entirety.
Often in a crowded underground utility environment of metallic pipes and cables, coupling of signals at the active locating frequency from the target conductor to other adjacent underground conductors can occur. These conductors (lines) are not intended to be tracked by the line location system, but coupling through various means (capacitive, conductive, or inductive) can lead a line locator astray such that the operator of the line location system ceases tracking the pipe or cable of interest and instead begins following an adjacent, nearly parallel line. A measurement of a signal direction parameter can effectively mitigate the effects of coupling and thereby allow the operator to detect situations where signals from an adjacent conductor are being detected and monitored in the receiver rather than signals from the target conductor.
When coupling occurs between adjacent lines, the induced voltage signal in the adjacent line is reversed from the signal present in the target conductor. This is because the current that has propagated to the adjacent line is seeking an easier return path to a ground stake at the transmitter of the line locating system. By convention, the outgoing signal from the transmitter is taken as the positive direction, and the incoming as the negative. By monitoring the signal direction in addition to signal strength, one can detect a likely coupling situation through a positive-to-negative direction change. Thus, an operator using a line locating device equipped with a signal direction measurement capability has an advantage over one who does not.
Even with this potential benefit, signal direction is not commonly present on line locating systems because the reliable determination of the signal direction is difficult. At least two methods are presently utilized to detect signal direction. Both methods require the collaboration of the transmitter to allow a phase reference to be derived at the receiver. With a common phase reference between transmitter and receiver, the signal direction can be deduced.
The first method, commonly called current direction, as described in U.S. Pat. No. 5,260,659 with additional development noted in U.S. Pat. No. 6,549,011, requires the transmission of harmonically related sinusoids (or as in the case of U.S. Pat. No. 6,549,011 sinusoids related by N*F1=M*F2, with N and M chosen from a special set of integers), and a compatible receiver with a convention that the signal direction is positive when the phases of each component sinusoid are as transmitted. For negative signal direction (indicating an incoming signal at the transmitter), the relative phase of the two component sinusoids switches to 180° for the same phase reference point at the receiver.
A similar approach to estimating the signal direction is described in U.S. Pat. No. 5,438,266. Two distinct and harmonically related frequencies are evaluated at the receiver to detect the reversal of the phase relationship between the two frequencies, and hence the change in signal direction.
The approaches detailed in the above methods of estimating signal direction rely on the fact that radio frequency (RF) wavelengths at these frequencies are long, and one can traverse a section of cable or pipe and be reasonably certain that a change in sign of the phase reference from positive to negative will be a result of the locating system picking up signals coupled to a parallel conductor instead of the target conductor. Unfortunately, there are several drawbacks to these approaches. The first problem is that the user must pause periodically and reset the phase reference to a new position, before enough distance has been traversed that the phase reference changes sign on the primary targeted conductor.
A second problem has to do with variability in the physical transmission medium of the cable or pipe, particularly at higher locate frequencies which are more susceptible to capacitive coupling. All such media are acknowledged in the communications literature as being characterized as a “channel,” with a measurable magnitude and phase characteristic as a function of frequency. Unfortunately, one cannot know this characteristic a priori, so the phase of the signal can change in unknown ways as the receiver is moved along the line. Because the two component signals can be separated from each other in frequency by a non-negligible factor, their relative phase responses can vary with position on the line. This compromises the ability to determine the phase reference point.
A third drawback to approaches that rely on the simultaneous transmission of disjoint frequencies is that the receiver hardware system becomes more complex in order to accurately process both component signals. For example, accurate discrimination of both frequencies requires doubling up the tunable analog filters.
The second method (referred to as signal select) is described in U.S. Pat. No. 6,411,073 and is attractive because no arbitrary phase reference needs to be set by the user at the receiving location. Instead, the transmitter and receiver collaborate by defining a phase reference in the transmitted signal via frequency modulation (FM). Modulating a small variation in frequency around a carrier adds one degree of freedom to the transmitted signal that can be unambiguously discriminated by the receiver independent of whether the signal direction is outgoing or incoming. This allows the adoption of a convention between transmitter and receiver that the phase of the carrier when the FM modulation is at the highest frequency defines the phase reference point, for the purpose of determining signal direction. For the primary targeted conductor, the receiver detects the same sense of the carrier at the high frequency deflection of the FM signal. However, for a parallel conductor that is carrying the signal in the reverse direction, the carrier sense is reversed at the peak frequency of the FM modulation.
Thus for the signal select method, the receiver can compare the carrier phase once per period at the FM modulation rate. This results in a frequent measure of signal direction that does not require the user to reset a reference phase, surmounting a flaw of the previous current direction measuring methods. Furthermore, since the FM modulation extends over only a small frequency range around the carrier (typically +/−1%) the phase response variation due to channel effects is small.
The elegance of this approach is offset by the difficulty of comparing signal phases as described in U.S. Pat. No. 6,411,073. As suggested, the FM modulation frequency is up to 100 times less than the carrier frequency. Thus the receiver must demodulate the FM signal, and compare to the carrier such that the phase error in FM demodulation can be no larger than ½ of the carrier signal period. This is equivalent to requiring an FM demodulation phase accuracy of 360/100/2=1.8°. An error larger than this will result in a false detection of signal direction. In environments subject to high interference, the FM demodulation accuracy is subject to increased error that can appear as phase jitter in the demodulated signal, reducing the reliability of the signal direction indication.
If the modulation frequency were increased, a less restrictive phase accuracy would be required for accurate signal direction estimation. However, this has a negative side effect of requiring an increased bandwidth at the receiver. Typically, line location systems reduce interference by implementing very narrowband filters around the carrier frequency, eliminating as much as possible signals picked up from the antennas that do not represent the active locating frequency. For the signal select method, the single sinusoidal carrier frequency has been extended to an FM modulated signal, and so the bandwidth of the locating system must necessarily increase to at least twice the modulation rate around the carrier. Thus the receiver, and the signal strength and signal direction estimators in the receiver, are open to more broadband noise and in-band interference than the normal (single carrier, non-FM modulated) case.
Therefore, there is a need for line location systems capable of accurately determining the signal strength parameter and the signal direction parameter from detected signals originating from a target conductor, especially at low signal levels in the presence of interference.